The present invention relates to a device for applying coating color to a fiber web. The device includes an application roll for applying the coating to the web, a nozzle which delivers the coating to the application roll, a trough to receive overflow coating and a device for cleaning the application roll surface including liquid application means and a scraper following the liquid application as the roll rotates.
Devices of this type can be arranged as part of or inside of or else outside of a paper manufacturing machine. The liquid to be applied may, for instance, be a size suspension with which the web of paper is to be impregnated, i.e. the size suspension penetrates more or less completely into the interior of the paper web. However, the liquid may alternatively be a coating composition meaning pigment suspensions of different types, from which a surface layer is formed on the web of paper. In both cases, the liquid can be applied merely to one side of the paper web or to both sides thereof.
Applicable prior art publications include:
1. Federal Republic of Germany Utility Model 8 414 413 and U.S. Pat. No. 4,848,268
2. U.S. Pat. No. 2,729,192
3. U.S. Pat. No. 2,946,307
4. U.S. Pat. No. 3,084,663
5. "Wochenschrift fur Papierfabrikation" 1973, pages 164 to 169.
6. "Wochenschrift fur Papierfabrikation" 1978, pages 773 to 778.
7. German Published Application DE-OS 35 12 892.
The invention proceeds from Reference 1, and particularly from FIG. 2 thereof. That discloses two application rolls which together form a nip through which a web of paper passes. Each of the two application rolls has a nozzle applicator associated with it which applies a film of coating color to the outer surface of the respective roll. Each nozzle applicator has a nozzle with two nozzle lips. The lip which is downstream in the direction of roll rotation has a free end which supports a roll doctor, i.e. a bar having a grooved outer surface. This enables a metered application of coating color to the outer surface of the roll.
Although it is not shown in Reference 1, in practice, a cleaning device is always present in order to clean the outer surface of each application roll. This cleaning device is located between the place where the film of coating color is transferred from the outer surface of the roll to the web of paper, i.e. the delivery position, and the place where the nozzle applicator delivers the film of coating color onto the outer surface of the roll, i.e. the feed position. Such cleaning devices are necessary for various reasons. On the one hand, particles of coating color and, on the other hand, fibers which are torn out of the web of paper upon its removal from the outer surface of the roll remain on the outer surface. These are stubborn deposits which accumulate upon each revolution of the roll and the deposits lead to non-uniformities in the film of coating color as well as to disturbances. Therefore, the deposits must be eliminated.
The known cleaning device comprises a scraper having a scraper blade which, in operation, is applied against the outer surface of the roll, and further comprises a source of liquid that applied liquid to the roll surface before or upstream in the path of roll rotation of the scraper blade contact with the roll surface. The source of liquid has two purposes. On the one hand, it sprays cleaning liquid onto the outer surface of the roll for loosening the deposits, or at least it prepares for the loosening thereof. On the other hand, it assures that at all times neither the scraper blade nor the subsequent doctor of the applicator nozzle travels dry over the roll surface. The blade and the doctor must be lubricated at all times. If this is not done, then the sensitive outer surface of the roll may be damaged by the scraper blade or the metering doctor.
Even before commencement of the coating operation, i.e. before the charging of the nozzle chamber with coating color, and thus before application of a coating film onto the outer surface of the roll, the liquid source must supply sufficient liquid in order to lubricate both the scraper blade and the doctor. The liquid source is generally a spray pipe which extends over the outer surface of the roll. The liquid is thus applied to the outer surface in excess quantity before commencement of the coating operation and the liquid remains adhering to the roll outer surface, passing through the slot between the scraper blade and the outer surface. A part of this adherent liquid then always continues to travel with the outer surface and arrives at the doctor, so as to also lubricate it. Another part of the liquid passes into the coating color collection trough. That trough is arranged in front of or upstream of the nozzle applicator in order to receive excess coating color. The trough therefore contains a mixture of coating color and cleaning liquid.
Once the coating operation has been started, and when the nozzle delivers a film of coating color in the desired amount to the outer surface of the roll, the metering doctor is lubricated by the coating color, and the doctor then does not require a separate lubricating liquid. It is then merely necessary to assure that the scraper blade receives sufficient lubricating liquid, generally water. The source of liquid thus need deliver less liquid after this time. Throttling of the amount of liquid being applied is, however, also necessary, since otherwise there is an undesired dilution and thus an impairment of the film of coating color.
The problem to which the invention is addressed starts here. It is difficult to adapt the following parameters to one another, namely the time of the commencement of the coating operation, i.e. the feeding of the film of coating color onto the outer surface of the roll, the time of the throttling of the feed of liquid, and the extent of the throttling.